The present disclosure relates to the field of borehole drilling for the production of hydrocarbons from subsurface formations. In particular, the present disclosure relates to fishing operations to recover tool strings.
An example of a fishing operation is the retrieval of a logging tool string that is stuck in a well. Logging tools may become stuck due to encounters with bridges, cave-ins, swelling formations, debris and the like. Often, an overshot device is used to enage the stuck tool string. During fishing operations, it is common practice to circulate mud or other substances down onto the stuck tools to clean the top of the fish that protrudes from the cable head of the tool string, and to determine when the overshot assembly engages the tool string. Typically, when the pump pressure increases, it is assumed that the pressure increase is due to the logging string being swallowed up by the fishing equipment overshot device. Currently, reconnecting the wireline during the fishing operation allows the logging engineer to monitor the down hole tension in order to determine when the drill string is pushing against the tool string and limit the weight that the driller puts down upon the tool string. In this way, the operator can recover the tool in a working condition and continue with the logging operation. It is important that the logging operation continue because the logging operation cost is based largely on the rig operation which is charged on an hourly basis and is generally quite expensive. Recovery of the tool reduces but does not eliminate the possibility that the tool string is dropped when the drill pipe is retrieved to the surface. The problem with the current state of affairs is that the tension increase that is seen by the logging engineer is only an indication that the drill string is pushing on the tool string. The tension increase is not, however, a complete indication that the overshot has actually swallowed the instrument itself or that the tool string is being engaged by the grapple within the overshot device. Instead, the tension increase could be the result of debris or other matter within the borehole itself that is preventing the instrument from being completely grappled by the overshot device.
If the tool string is not properly engaged by the grapple, the tool string may be dropped while the drill pipe is being retrieved to the surface. Unfortunately, dropping the tool string is a familiar occurrence during fishing operations. The weight of the logging tool string is light by comparison to the drill string, and therefore the driller often does not sense the weight of the logging tool string on his weight indicator because it is so small in comparison to the rest of the equipment. Moreover, once the tool string is engaged, circulation is generally impossible or only possible when a pump out sub is run for the express purpose of providing circulation after the fish is engaged. In many cases it is not recommended to circulate because the possibility exists that the fishing neck of the tool string is not properly engaged and the circulation can push the tool string out of the fishing equipment and be lost once again downhole. There is, therefore, a need in the art for some mechanism to ensure proper engagement with the tool string with the overshot device.
While the present invention is susceptible to various modifications and alternative forms, specific exemplary embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.